Is this with requesting tracks? Wouldn't be able to tell you much about that error, but I asked foreversonic and he seems to think it's when similar games have been requested by others who've requested en masse.
As for The Hybrid Front, we can look into it, but that's not been touched for a while so I don't know what the problem with that is.
Yeah, just had a look, and a 603 error seems to be due to a song already in a queue to be played. Seeing as users can make up to 15 requests at a time, and some tend to bulk request, I would imagine that's why certain songs are being blocked out.
Erm... Not being rude Gavvie, but when I get the error I'm normally the only person requesting at the time and I never see the tracks play after. So I think it's something else.